Decontamination of sub-systems of LWR plants has now become relatively common in the United States and is important as a useful contributor to the reduction of radiation exposure of workers at these plants. Sub-system decontamination involves exposing a part of the reactor circuit to chemical decontamination solutions which dissolve radioactive deposits which have accumulated on process equipment which includes piping. The spent decontamination solutions may then be treated by ion exchange to retain the chemical and radioactive burden of the decontamination solution on the resin, while clean water is returned to the system. An example of such a process is the LOMI process, described in U.S. Pat. No. 4,705,573.
One of the purposes of decontamination is to remove the radioactive deposits which can represent a danger to plant workers. Decontamination of plant components which are intended to be returned to service should avoid any damage to materials exposed to the process. Such damage could occur due to corrosion resulting from the process or from normal operating conditions of the nuclear plant subsequent to decontamination. Certain processes which attempt to avoid damage do not attack base metal and operate by dissolving the overlying layer of corrosion product metal oxides.
Although effective in lowering or reducing the amount of radiation to which workers are exposed, such processes do not remove all radioactivity from treated surfaces and are therefore not capable of allowing the plant items to be treated as non-radioactive waste. In order to sufficiently decontaminate radioactive items to be able to classify them as non-radioactive, it is necessary to remove a thin layer of the underlying base metal, so as to release radioactivity trapped in fissures in the metal (occurring, for example, as a result of mild intergranular attack of the metal surface.) For decommissioning a reactor, restrictions concerning plant damage are not as stringent since the plant items are not required for further operational duty. The only requirement with regard to damage is that the plant items maintain their integrity against leakage during the operation of the process while remaining structurally sound. Although the removal of a thin layer of base metal is consistent with these requirements, removal of too much metal may cause a problem concerning the amount of radioactive waste generated.
Several processes have been described for removal of base metal. For example, U.S. Pat. No. 4,828,759 is directed to a process for using fluoroboric acid as a decontaminating reagent. The reagent is capable of dissolving a wide variety of metals and metal oxides. The patent details several methods for using the acid to minimize radioactive waste, for example, recovering the acid by distillation. The process described may be convenient for treating components which are immersed or sprayed in a bath for decontamination. The concentration of acid stated (0.05 to 50 moles per liter) is sufficiently great to avoid the complications of ineffectiveness referred to below.
In some instances, using a dilute chemical system may be advantageous when decontaminating large components of nuclear plants, such as steam generators. The purchase and handling of chemicals is difficult and expensive if concentrated chemical solutions are used, and it is difficult to manage the wastes in a minimum volume. Although a process described in U.S. Pat. No. 4,828,759 overcomes many of these difficulties, the type of equipment proposed is not commonly used in a temporary manner in nuclear plant decontamination, and the process does not easily allow the benefits of exposing the items to be decontaminated to a progressively cleaner decontamination solution. Use of progressively cleaner decontamination solutions is useful for obtaining high decontamination effectiveness in a large convoluted system of plant items contaminated on inaccessible internal surfaces.
Another decontamination solution capable of dissolving base metal involves cerium salts in an acid solution (e.g. German Patent No. DE-PS 2, 714,245). The oxidizing action of cerium (IV) in conjunction with a mineral acid such as nitric acid causes the metals to be dissolved. The cerium (III) resulting from oxidation of the metal can be reoxidized to cerium (IV) by the action of an oxidizing chemical such as ozone. The problem with systems based on cerium as oxidant is that cerium is cationic and is removed and depleted along with metals and radioactivity by ion exchange. It is therefore difficult to provide a system that allows continuous removal of cationic radioactive metals without consequent removal of cerium. The desired objective of treating the system with a progressively cleaner decontamination solution cannot therefore be accomplished conveniently.